Human Rights Watch today accused the Islamist movement Hamas of a campaign of killing and attacks against Palestinians in Gaza that has left at least 32 dead and dozens more seriously injured.

The attacks came over the past three months, beginning during Israel's three-week war in Gaza. "Hamas authorities there took extraordinary steps to control, intimidate, punish and at times eliminate their internal political rivals as well as persons suspected of collaboration with Israel," Human Rights Watch said in a report released today.

During the Gaza war 18 Palestinians, many suspected of collaborating with Israel, were killed. Most had escaped from Gaza's main prison after it was bombed by Israeli aircraft at the start of the war. A further 14, at least four of whom were in jail at the time, have been killed since the end of the war.

"The unlawful arrests, torture and killings in detention continued even after the fighting stopped, mocking Hamas's claims to uphold the law," said Joe Stork, deputy director of Human Rights Watch's Middle East division.

It was the worst internal Palestinian violence inside Gaza since Hamas, which won Palestinian elections three years ago, seized full control of the strip in mid-2007.

The accounts corroborate witness testimony reported by the Guardian at the time and appear to show Hamas took advantage of the chaos of the war to exert control over its political and security rivals in Gaza. Other Palestinians have also spoken of a campaign of intimidation against secular and moderate groups in Gaza.

Human Rights Watch said the extent and frequency of the attacks "strongly suggests" they resulted from a policy decision by Hamas leaders.

Palestinian human rights groups in Gaza also found 49 Palestinians were shot in the legs in punishment attacks and around 73 were severely beaten, suffering broken arms and legs, from the start of the war in late December until the end of January. Some of the attackers were not identified, but many appeared to be from Hamas.

Hamas told Human Rights Watch charges had been brought against police over two deaths and that another two were being investigated. "Four investigations into 32 deaths are not enough," Stork said.

Taher al-Nunu, a Hamas spokesman in Gaza, denied Hamas forces were involved in extrajudicial killings. He said: "Some resistance factions claimed they had killed suspected collaborators during the Israeli war on Gaza and in areas where battles took place, and we have launched an investigation which has not yet been completed."

Human Rights Watch said Fatah, the rival, western-supported Palestinian faction that controls the West Bank, had also used "repressive measures" against its Hamas opponents. It said Palestinian human rights groups recorded 31 complaints of torture by the Fatah-led security forces, as well as one death in custody and the arbitrary arrest of two Palestinian television journalists.

"Western governments that support and finance the Fatah authorities in the West Bank have remained publicly silent about the arbitrary arrests and torture against Hamas members and others," said Stork.

In the most recent incident, on Sunday, a Hamas MP and preacher, Hamed Bitawi, was injured after he was shot in the leg by an officer from the Fatah-led security forces in the West Bank city of Nablus.

Human Rights Watch has also accused Israel of violating international law during the Gaza war, including by what it said was indiscriminate use of weapons such as white phosphorus in densely populated civilian areas.